util: remove code spawning with systemd activation env vars

util: remove code spawning with systemd activation env vars

The only use of this code was removed by:

  commit be78814ae07f092d9c4e71fd82dd1947aba2f029
  Author: Michal Privoznik <mprivozn@redhat.com>
  Date:   Thu Apr 2 14:41:17 2015 +0200

    virNetSocketNewConnectUNIX: Use flocks when spawning a daemon

less than a year after it was first introduced in

  commit 1b807f92dbb617db5b9d551777d3026d8ff0903f
  Author: Martin Kletzander <mkletzan@redhat.com>
  Date:   Wed Jul 16 08:00:19 2014 +0200

    rpc: pass listen FD to the daemon being started

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>